Semiconductor workpieces are often implanted with dopant species to create a desired conductivity. For example, ion beams comprising ions containing the dopant species are accelerated toward a workpiece to implant the dopant species into the workpiece.
In ion implantation system, it is often necessary to measure the cumulative ion dose that is implanted in the semiconductor workpiece. The ion dose determines the effective conductivity of the implanted workpiece.
Typically, the ion beam current is measured by current measuring devices, which may be located in certain locations in the ion beam path. For example, in one embodiment, faraday cups are utilized. A faraday cup is made using a conductive material, typically formed having sides, a bottom and an open top, which may resemble a cup. Ions that enter through the open top strike the conductive material, thereby imparting their charge on that conductive material. The charge imparted on this faraday cup is then measured. This charge, when viewed in conjunction with the surface area of the bottom of the faraday cup, can be used to measure the density of the ion beam. This density can then be multiplied by the actual area of the ion beam, or the area of the workpiece, to determine the overall ion beam current being imparted on the workpiece.
This process is effective in cases where the width of the ion beam is greater than the width of the workpiece, as shown in FIG. 1. In this figure, the faraday cups 20 can be located just beyond the perimeter of the workpiece 30. The ion beam 10 strikes the workpiece 30, and also strikes the faraday cups 20 located outside the workpiece 30. In addition to requiring an ion beam 10 that is wider than the workpiece 30, this technique requires that the ion beam be relatively uniform, as the ion beam current sampled by faraday cups 20 located outside the workpiece 30 are assumed to be representative of the ion beam current that is implanting the workpiece 30.
As workpieces continue to grow in size, the amount of space available to locate faraday cups continues to decrease. In addition, the wider ion beams needed to implant these larger workpieces may not have an ion beam current that is as uniform as was previously achieved. This may be especially true with respect to workpieces having a width of 450 mm or greater.
Therefore, an improved system and method of measuring ion beam current in a process chamber would be beneficial.